Field of the Invention
This invention relates to the manufacture of large structural members such as steel girders for use in bridges or other structures and, in its preferred embodiment, is especially adapted for use in performing certain drilling operations required in the manufacture of such structural members.
The class of structural members with which the apparatus of the invention is primarily intended for employment are typically fabricated of steel in a generally I-shaped cross-sectional configuration involving a pair of side plates centrally inter-connected by a transverse web (and, perhaps, additional bracing elements), presenting at each side of the members what are commonly referred to in the trade as "flanges" extending in opposite directions from the central web. Such members may typically have lengths in the range of about 50 to 100 feet, widths in the range of about 4 to 12 feet, side plate widths in the range of about 10 to 30 inches and flange thicknesses in about the range of 3/4 to 21/2 inches. Such members are obviously of corresponding weight and difficulty as to handling.
In the construction of bridges or other structures in which such members are used, it is typically required that a plurality of such members will be joined together in end-to-end abutting relationship to provide a single, relatively rigid, elongate structural assembly of the final structure being constructed. Such joinder between abutting end portions of adjacent members in such an assembly is normally accomplished by providing steel plates of suitable dimensions, up to about 3 feet by 11/2 feet being typical, in overlapping relationship with the end portions of both of the adjacent structural members, on both the inner and outer surfaces of the flange thereof, and then securing such splice plates to the flanges of the members by a plurality of relatively large bolts passing through the outer splice plate, the flange of the structural member and the inner splice plate with a securing nut then emplaced upon each bolt. The splice plates will typically have a thickness within the range of about 3/8ths to 11/2 inches, so that the overall thickness of the outer splice plate, the inner splice plate and the flange of the structural member sandwiched therebetween will typically aggregate to several inches. The diameter of the bolts employed in thus joining girders or comparable structural members will typically be in the range of about 3/4 inches to 1 inch. In order for the bolts and various portions of the splice plates to appropriately share the shear and other forces involved in accomplishing a rigid interconnection between the longitudinally abutting structural members, it is necessary that the bolts be received within holes in the splice plates and the flanges with relatively limited clearance tolerances.
It is impractical, of course, to attempt to drill the holes required for accommodating the fastening bolts during actual assembly of the structural members into the bridge or other final structure being constructed, since such members will then be typically at an elevated location substantially above water or ground level requiring that operations to be performed by the assembly workmen must be minimized, and it will also be appreciated that the equipment required for drilling in the type of work-pieces involved is necessarily relatively bulky and heavy to an extent rendering it virtually impossible for workmen to properly handle such equipment in such an environment. Accordingly, it is virtually essential and customary that the sets of holes required in both the structural members and the splice plates will all be pre-drilled, so that the splice plates and bolts may simply be assembled with the structural members during final assembly of the bridge or other structure. Since a typical bridge, for example, may require the employment of several thousand bolts for joining structural members together with splice plates, it will be apparent that a very substantial number of pre-drilling operations will be required. Moreover, unless the individual holes and flanges of each pair of abutting structural members and the holes in the splice plates to be associated therewith are positioned in near perfect alignment with each other, it will be manifest that substantial difficulties and delays will be encountered during final assembly of the overall structure by the inability of assembly workmen to install the required bolts without the reworking or substitution of parts.
Accordingly, the customary practice is that the structural members to be thus interconnected during assembly of the final structure will first be laid out generally horizontally, but in their desired final relative juxtapositions, upon "horses" or other suitable supports in a fabrication yard in an undrilled condition, the splice plates in undrilled contion to be associated with such structural members will also be placed and clamped into their desired final relationships to the latter, and the required holes will then be drilled through the splice plates and flanges of the structural members to be joined in one drilling operation, thereby assuring alignment of the holes through all three of such elements for each bolt to be inserted therethrough. The ends of the particular structural members to be joined in the assembled final structure and the particular splice plates to be employed in accomplishing that joinder will normally be appropriately marked to facilitate their being properly associated with each other during final assembly of the structure.